1. Field of Invention
Embodiments of the invention exemplarily described herein relate generally to systems and methods for forming gas hydrates and, more specifically, to a system and method of forming gas hydrates wherein gas hydrates are removed from a reactor concurrently with the formation of such gas hydrates within the reactor.
2. Description of the Related Art
Gas hydrates have a non-stoichiometric crystalline structure that consists of low molecular weight gas molecules (i.e. CH4, C2H6, CO2, etc) and host water molecules. In gas hydrates, gas molecules are encaged within water cavities. Because of this structure, water molecules are called ‘host’ molecules and gas molecules are called ‘guest’ molecules. The stoichiometric formula of gas hydrates is Gas (H2O)n, where n is the hydration number. The hydration number is usually 5 to 8 in a water cavity. The water cavity structure is maintained via hydrogen bonds between water molecules under the guest gas environment. Typically there is van der Waals bonding between a non-polar gas molecule and a host water molecule.
The current reserve of natural gas in hydrate sediments provides a tremendous potential as a future energy source (estimated at 104 giga-tons (Gt) of carbon and this amount exceeding all other fossil fuel deposits (5,000 Gt) on earth). Gas hydrates are known to exist as one of three structures; structure I, structure II, and structure H. Structure I is a body centered cubic structure; these hydrates are generally present in the permafrost region and in deep oceans. Structure II is a diamond lattice formed with the gases that are bigger than ethane and smaller than pentane. Structure H has three different cavities with square, pentagonal and hexagonal faces while structures I and II form two types of cavities; pentagonal dodecahedron (small cavity), tetrakaidecahedron (structure-I large cavity) and hexakaidecahedron (structure-II large cavity).
Gas hydrates were first discovered by Humphry Davy in 1811. In the mid 1930's the importance of gas hydrates was emphasized after Hammerschmidt discovered that gas hydrates were responsible for plugging natural gas process and transportation lines. For a long time, research in the petroleum industry had been focused on avoiding the formation of gas hydrates.
In addition, gas hydrates serve as a good medium for storing and transporting natural gas and hydrogen. Methane hydrates hold more than 160 volumes of methane gas per unit volume of hydrate at a standard state condition (0° C., 1 atm). The high concentration of gas in the hydrates has led researchers to consider intentionally forming these materials for the purpose of storing and transporting natural gases more safely and cost effectively. Many bulk gas hydrate formation processes have been proposed in which aqueous solution and gas are supplied within a reactor and maintained under a temperature and pressure sufficient to induce the formation of gas hydrates within the reactor. After gas hydrates have been formed, the reactor is then shut down and the gas hydrates are removed from the reactor. Removal of gas hydrates from a bulk reactor can be time consuming due both to the kinetically slow processes under which gas hydrates are formed and to the need to shut down the bulk reactor to remove gas hydrates that have been formed. Moreover, gas hydrates are solid, bulky masses which stick to interior surfaces of the reactor and other structures within the reactor (e.g., sensors, blades, etc.) and are difficult to transport. Recognition of these and other problems and limitations of convention hydrate forming systems that provided the impetus for the present invention.